Thanks to the first comment in the last post which directed me to a math problem (http://contestcoding.wordpress.com/2013/03/22/palindromic-primes/) . From a scientist point of view, the answer itself, 98689, is just simple, but the way to answer it is kind of interesting. Let’s have a look at it step by step how a scientist solved it.

1. How to generate a list of palindromic number?

‘A palindromic number is a number that reads the same backwards as forwards (1991 for example).’ One can iterate in all numbers and check if it is palindromic:

str(num)==str(num)[::-1]

, or, a smarter way to generate them [1]:

from itertools import product def palindromeNum(n): return [n*'%s'%tuple(list(i)+list(i[n*(n-1)/2%(n-1)-1::-1])) for i in product(*([range(1,10)]+[range(0,10)]*((n+1)/2-1)))]

2. Now we have a list of palindromic number, how do we determine it is a prime or not?

Simple. The is_prime() function from textbooks should work with no problem. One can loop from 2 to the square root of N and see if it can divide N. Let me introduce you a better way, Miller Rabin primality test, a pro’s way.

wikipedia: http://en.wikipedia.org/wiki/Miller%E2%80%93Rabin_primality_test

In general, it is a faster and more efficient way to test if a number is a prime. Some other methods can be AKS primality test (wikipedia) and so on. It is good to know them 🙂

There is a nice implementation of Miller Rabin in python here, one can have a look at this beautiful algorithm.

So, we can build a loop from the end of the list of palindromic number that we generated at step 1, and test each of them using Miller Rabin primality test, we can easily find the number is 98689.

3. Some additional thoughts

- The problem asked about 5 digits palindromic number. Also, these kind of numbers exist for 3 digits, 7 digits, 9 digits and so on, but never exist for 4 digits, 6 digits. Can you guess why? hint: some thing about number 11.
- There is a nice website http://oeis.org/A114835 for the set of palindrome prime numbers. There are some other interesting sets of numbers.
- Do you know the largest 11 digits palindromic number is 99999199999, 13 digits 9999987899999?

Source code:

Actually the function palindromeNum() can be optimized by generator and yield, which does real time calculation rather than unnecessarily generating the whole list.

from itertools import product import miller_rabin def palindromeNum(n): return [n*'%s'%tuple(list(i)+list(i[n*(n-1)/2%(n-1)-1::-1])) for i in product(*([range(1,10)]+[range(0,10)]*((n+1)/2-1)))] for i in palindromeNum(5)[::-1]: if miller_rabin.miller_rabin(int(i)): print i break